Calcium and Magnesium are the most utilized and required micro nutrient and some botanist consider them macro nutrients. Short answer, due to the abundance of red and blue light from leds chlorophyll a and b are the primary pigments used to force the electron jump required to produce energy from light, air and water. Chlorophyll a and b need calcium and magnesium in abundance in order to make this happen if they are the only pigments being used. Normal round light provides the wavelengths needed to put cartenoids, phycoerythrin, and phycocyanin (there are more pigments but I'm not going to look them up right now) to use inside of the leaf in order to produce the electron jump to create energy. Therefore when using round light you will have a normal requirement for calmag, but if you only focus your nm toward the chlorophyll spectra you will need additional calmag in order for your plant to replenish depleted chloroplasts in the leaves. This is just a general over view. If you want more info do some research into chloroplasts and chlorophyll. The internet is a wonderful place if you know where to look.
http://en.wikipedia.org/wiki/Chlorophyll#Chemical_structure You will notice that each molecule of chlorophyll needs mg.
Often in the modern era of leds you will see most manufacturers putting in a few 4,000k - 10,000k diodes into their light for safe measure, if not just going with an all 4500k/ 660nm spectra blend of leds. Not all breeds of cannabis have the same requirements for light, nutrients, and water. It's best to study your plants and play it out on a case by case basis. I hope this helps.
Sincerely,
ILovePlants
